Climate Change: What Put the Hiatus in the Global Warming Trend?
by Bill Chameides | September 27th, 2011
posted by Erica Rowell (Editor)
Further insights into stalls in global warming.
From a long-term, multi-decadal view, global temperatures have been on the rise, and the evidence is quite strong that human activities — and more specifically emissions of carbon dioxide (CO2) and other greenhouse gases — are largely responsible. But there are aspects of the global temperature trend that remain unexplained or at least not well explained.
The Hiatus Mystery
A case in point is the year-to-year changes in global temperatures and in some instances changes in global temperatures over periods as long as a decade or so. Take for example the period from 2000 to 2009. It was a warm decade, in fact the warmest 10-year period on record. But the temperature variations over that decade are curious. Curious because there was no significant variation; in effect global warming took a hiatus.
Such hiatuses are not without precedent; in fact, if we look at the temperature record of the past 100+ years (see graphic), we find a number of instances of other global warming hiatuses superimposed on the long-term rise in global temperatures: for example, the periods between roughly 1900–1910, 1930–1949, 1980–1990, and the long mid-century hiatus from 1950–1970.
The appearance of such hiatuses raises a problem; it seems to violate what is arguably the most fundamental principle of physics — namely, the first law of thermodynamics, which states that energy is conserved — and violating that law cannot be.
- The observed rise in global atmospheric temperatures (and also, by the way, the temperatures of the ocean’s upper layers) is indicative of the fact that the energy content of the atmosphere and ocean is increasing.
- That increase in energy must come from someplace according to the first law. Where? From the extra bit of energy being trapped in the atmosphere by the increase in greenhouse gases. Climate scientists have sussed that out pretty well, producing a balanced energy budget for the Earth with the extra heat from greenhouse gases matched by increased energy in the atmosphere and surface ocean.
- During a hiatus, greenhouse gases continue to trap extra heat, but the atmospheric and surface ocean temperatures don’t increase. Where does the extra energy go?
Hypotheses Already on the Table: Add Cooling
The curious hiatus of 2000–2009 has piqued climate scientists to try to answer that question, and a number of researchers have thrown their ideas into the ring of hypotheses. So far, most of these have attacked the issue by questioning the basic premise of the conundrum that there is an energy imbalance in the first place. More specifically, they argue that other factors come into play during the hiatuses that temporarily cancel out the warming from greenhouse gases — in others words, factors that add in a little cooling. These other cooling factors include:
- A decrease in stratospheric water vapor,
- An increase in the activity of small, tropical volcanoes,
- An increase in particulate pollution from coal burning in China and other developing countries, and
- An especially strong solar minimum.
New Paper Suggests Other Sink for the Heat
Last week a paper published in the journal Nature Climate Change by Gerald Meehl of the National Center for Atmospheric Research and colleagues advanced a new hypothesis that does not involve extra cooling.
Their hypothesis instead posits that the extra energy being trapped by greenhouse gases goes into the deep ocean during these hiatus periods instead of the atmosphere and upper ocean. To show this, they used an ensemble of five climate models simulating the 21st century. While the average of the five simulations produced a monotonically increasing global temperature (indicative of a long-term trend), the temperature variations for the individual models were a good deal more complex and included periods when the global warming trend in a given model took a hiatus.
Looking more closely at a subset of these, the authors found that during each hiatus, atmospheric and upper ocean temperatures remained relatively flat and the extra energy trapped in the system showed up in the mid-level to deep ocean. In other words something happens to the ocean circulation during these periods that causes the deeper ocean to suck up more energy from the upper ocean (and in turn the atmosphere) — the result: a temperature hiatus.
Intriguing. Two questions arise:
- Is it possible to test this hypothesis using direct observations? Difficult, say Meehl et al, because of our limited ability to monitor the heat content of the deeper ocean.
- What causes the deeper ocean to gobble up all that heat during these hiatus periods? One possible explanation the authors hint at is that a large atmospheric-ocean circulation pattern like the El Nino-La Nina oscillation might be at work — an idea consistent with a recent paperby Boston University’s Robert Kaufmann et al.We know that global temperatures tend to rise more steeply during El Ninos and less during La Ninas. We also know that over the past decade the ocean circulation has tended more towards La Nina than El Nino; the ‘90s on the other hand tended to be dominated by El Nino conditions. Whether the timing between La Nina-dominated periods and hiatuses in warming is a coincidence or an indication that both are part of a larger circulation phenomenon affecting the global ocean isn’t yet clear.
The hiatus mystery has triggered a good deal of interesting research and we’ve now got an intriguing and growing set of hypotheses to choose from. Which one am I betting on? Not sure. The one bet I’m not making is that the global temperature hiatus is anything more than that — a hiatus. I’ll shake on that.filed under: carbon dioxide, carbon dioxide emissions, climate change, coal, El Nino, El Nino-Southern Oscillation, faculty, global warming, La Nina, oceans, temperatures
and: climate models, cooling, El Nino, El Nino-Southern Oscillation, first law of thermodynamics, greenhouse gas emissions, greenhouse gases, La Nina, particulates, solar cycle, solar energy, solar minimum, volcanoes, water vapor